Plant-based insect repellents

Topical repellents do not kill and mostly do not touch the insect. They interfere with how it finds you, which makes them a problem in olfaction and in physical chemistry.

What the mosquito is following

Host-seeking is a staged behaviour, each stage cued differently. Carbon dioxide in an exhaled plume acts at long range and switches the insect into an active search. Closer in, skin volatiles provide the specific signal — lactic acid, ammonia, short-chain carboxylic acids and 1-octen-3-ol, in ratios that differ between people and are partly determined by skin bacteria. At short range, body heat and water vapour guide the landing.

A repellent has to disrupt one of these stages before contact. Two mechanisms are described: interference with olfactory receptor signalling, so the host cues are masked or the signal is confused, and direct avoidance, in which the insect detects the repellent itself as aversive. For DEET both have been argued and the balance remains debated; for the plant terpenoids, receptor-level interference is the usual proposal.

Volatility is the whole trade-off

The consequence is that the active must be in the vapour phase above the skin to work. It is not a barrier and it does not need to be touched.

That requirement is also the limit. A molecule volatile enough to maintain a vapour layer is volatile enough to be lost from the skin, and the loss is accelerated by warmth, wind and sweating — exactly the conditions under which repellent is used. Protection duration is therefore governed by vapour pressure, and cannot be separated from efficacy. An active made less volatile lasts longer and repels less well.

Monoterpenes such as citronellal, geraniol and limonene sit at the high-volatility end. Their vapour pressures are far above DEET’s, so they establish a vapour layer quickly and lose it quickly, which is the physical reason typical essential-oil repellents give roughly twenty minutes to an hour rather than several hours. Formulation can slow the loss — microencapsulation, polymer film formers, higher-boiling co-solvents — but it works against the same trade-off, since anything that slows release lowers the headspace concentration.

The PMD distinction is the one to get right

Oil of lemon eucalyptus, as a registered repellent active, is not lemon eucalyptus essential oil. The essential oil of Corymbia citriodora is dominated by citronellal. Treating that oil with acid cyclises the citronellal into para-menthane-3,8-diol (PMD), a heavier, much less volatile diol, and it is PMD — enriched to a defined level — that carries the activity.

The difference is exactly the volatility trade-off made deliberately. PMD’s lower vapour pressure gives it the several-hour protection that puts it in the same duration band as lower-concentration DEET, and it is the only plant-derived active registered as a skin-applied repellent by the US EPA and recommended alongside the synthetics by the CDC. The unrefined essential oil does not carry that recommendation, and the two appear under similar names on labels.

The claims that do not follow

Wearable patches and area devices face a physical objection rather than a chemical one: a vapour source not on the skin protects the volume around it, and any air movement displaces that volume. Field evaluations of citronella-based wristbands and patches have generally found little to no protection at the skin surface.

Duration claims deserve the most scepticism, because the standard measurement — complete protection time, the interval to the first confirmed bite in a controlled arm-in-cage or field test — is well defined, so a product not quoting it is choosing not to.